1. Field of the Invention
This invention relates to a magnetic resonance imaging method and a magnetic resonance imaging system.
2. Background Art
Magnetic resonance imaging (MRI) is a method for imaging echo signals generated from atomic nuclei such as protons in magnetic resonance by collecting positionally encoded echo signals using gradient magnetic field and by decoding using Fourier transform.
In general, it takes much time for imaging by MRI, and various proposals have been presented for reducing the imaging time.
EPI (echo planar imaging) is most representative one of high-speed MRI imaging methods. EPI is a method for collecting echo signals necessary for filling a k space as a multi-echo by switching (inverting) a gradient magnetic field very quickly in a uniform magnetostatic field generated by excitation by RF pulses.
EPI uses multi-shot switching or single-shot switching for switching of a magnetic field. Multi-shot EPI collects echo data necessary for reconstructing images over a plurality of occurrences of excitation whereas single-shot EPI collects all echo data in one step of excitation.
Since single-shot EPI enables obtaining MR images in a very short time as short as 30 to 40 ms, it is also used in functional MRI (FMRI) for catching dynamic changes of viscera, especially, images of blood flows in brains or hearts, peristalses of cardiac muscular walls and digestive tracts.
However, in case of imaging by the EPI method, read-out magnetic gradients are inverted in polarity. Therefore, when using the conventional method configured to reconstruct images by simply assembling obtained echo signals, single-shot EPI images are subject to ghost images called artifacts, and invite diagnostic errors.
Especially in FMRI which takes hundreds of images successively, an eddy current generated during imaging is liable to cause changes in intensity and phase of ghost, this is added to original images and varies the signal intensity of images together with the ghosts. This was a bar to quantitative analysis in FMRI.